Method of bleaching cellulosic materials



Patented Apr. 4, 1950 METHOD OF BLEACHING CELLULOSIC MATERIALS Karl Nieolaus Cederquist, Falun, Sweden, al-

signor, by mesne assignments, to

Mathiclon Chemical Corporation, a corporation of Virginia No Drawing. Application April 22, 1946, Serial No. 663,962. In Sweden November 17, 1945 4Claima.

This invention relates to improvements in the bleaching of cellulosic materials such as paper pulp, prepared by the sulphite, sulphate or soda process, regenerated cellulose, rayon, such as viscose or cellulose acetate, cotton and other vegetable materials.

It has previously been proposed to bleach pulp by introducing chlorine dioxide into an alkaline solution in which the pulp is suspended. Said method comprises suspending the cellulosic material to be bleached in an aqueous solution having distributed therethrough an inorganic alkaline material of the class consisting of the alkali metal and the alkaline earth metal oxides, hydroxides and carbonates, introducing chlorine dioxide into said aqueous solution, and removing the bleached cellulosic material from the solution. The chlorine dioxide may be added as a gas or as an aqueous solution. The method may also be used for completing the bleaching of a cellulosic material which has been partially bleached, for instance by means of an acidified chlorite solutio r In said bleaching operation, at least under certain conditions with regard to alkalinity and reducing agents, chlorite is formed. On account hereof the introduced chlorine dioxide has only partially been made use of in the bleaching. It has been proposed to use the removed chloritecontaining liquid for a precedent bleaching of a subsequent batch of pulp after activating the liquid by acidification or by means of chlorine.

It has been proposed to bleach the pulp partially with chlorine in an aqueous medium and to complete the bleaching with a preferably acid aqueous solution of a chlorite. It has also been proposed to bleach cellulosic material by subjecting it to the simultaneous action of chlorine and chlorite by introducing chlorine gas in the chlorite solution or by forming chlorine water and adding it to the chlorite solution.

I have discovered that certain difficulties arise in obtaining a sufilcient whiteness in bleaching with chlorine dioxide under alkaline conditions and that a considerably more effective bleaching may be obtained by interrupting the introduction of chlorine dioxide at a suitable time before the operation is completed, and thereupon directly acidifying the chlorite-containing bleaching bath and complete the bleaching with chlorite, or, with or without addition of acid, activating the chlorite bath with chlorine and completing the bleaching with chlorine plus chlorite.

In order to perform the bleaching in the best manner and with a minimum amount of chlorine dioxide the addition of chlorine dioxide should be interrupted at the moment such an amount of chlorite is formed that the said chlorite after being acidified or activated with chlorine can accomplish the bleaching so that a substantially complete or desired whiteness of the pulp is obtained. If necessary the amount ofchlorite may be adiusted by a suitable addition of solid or dissolved chlorite from storage.

The temperature at the bleaching may be varied over a wide range (from 35 to 100 0.). It can be constant during the whole bleaching operation or it may be higher or lower during the final bleaching than during the addition of chlorine dioxide. For instance chlorine dioxidecontaining gases may be introduced into the alkaline bath at C. and the final bleaching be performed after acidifying with hydrochloric acid at C.

The bleaching method described above may be used alone in one or several steps or in combination with other bleaching methods. For instance a precedent bleaching of the pulp may be performed with chlorine and/or hypochlorite in two or several steps and a completing of the bleaching be accomplished with chlorine dioxide in the above mentioned manner.

It is true that it has been proposed to bleach with chlorine dioxide in an alkaline solution, to bleach with chlorite in an acidified solution and also to bleach with a chlorite solution activated with chlorine, but my method described above which has been found to give a sufilcient whiteness and strength of the pulp is a novel combination of the said bleaching methods and differs in essentials from these or from known features of bleaching with chlorine dioxides and chlorites.

The advantages of my method in relation to conventional bleaching methods may be stated as follows:

1. There will be no loss of chlorine dioxide by reason of the ready and complete absorption of the gas in the alkaline liquid also at higher concentrations which will not be the case if the absorption is performed in a neutral or acid solution.

2. A sufilcient whiteness may be obtained by completing the bleaching with the chlorite formed, which is not the case in bleaching with only chlorine dioxide in an alkaline solution.

3. The formation of chlorite in the alkaline solution occurs substantially quantitatively and without the .formation of chlorate on account of the presence of reducing agents in the pulp which are oxidized, whereby the pulp is bleached.

This results in the same bleaching eilect being obtained with a smaller amount of chlorite than is necessary in the case the bleaching bath is charged with finished chlorite, the chlorine dioxide having, already during its partial reduction to chlorite, oxidized part of the colored compounds of the pulp. Furthermore, the advantage is gained that the bleaching is partially performed directly by means of chlorine dioxide.

Having thus described my invention I declare that what I claimis:

1. Method of bleaching a cellulosic material comprising suspending the material in an aqueous alkaline liquor, charging chlorine dioxide to the liquor with the chemical make up of the liquor being such as to result in the conversion of a part of the chlorine dioxide to a water-soluble chlorite, until the material is partially bleached and until sufficient chlorite is formed in the liquor to complete the bleaching and completing the bleaching of the partially bleached material while remaining in the liquor by activating the chlorite formed therein during the partial bleaching.

2.'Method of bleaching a celluloslc material comprising suspending the material in an aqueous liquor containing a compound of the group consisting of the alkali and alkaline earth metal 3. Method of claim 2 where the chlorite isactlvated by acidification of the liquor.

4. Method of claim 2 where the chlorite is activated by the addition of chlorine to the liquor.

KARL NICOLAUS CEDERQUIST.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,100,496 Taylor Nov. 30, 1937 2,129,719 Vincent Sept. 13, 1938 2,195,302 Hampel Mar. 26, 1940 2,203,205 Rawling June 4, 1940 2,203,212 Casciani June 4, 1940 2,377,171 Murdock May 29, 1945 

1. METHOD OF BLEACHING A CELLULOSIC MATERIAL COMPRISING SUSPENDING THE MATERIAL IN AN AQUEOUS ALKALINE LIQUOR, CHARGING CHLORINE DIOXIDE TO THE LIQUOR WITH THE CHEMICAL MAKE UP OF THE LIQUOR BEING SUCH AS TO RESULT IN THE CONVERSION OF A PART OF THE CHLORINE DIOXIDE TO A WATER-SOLUBLE CHLORITE, UNTIL THE MATERIAL IS PARTIALLY BLEACHED AND UNTIL SUFFICIENT CHLORITE IS FORMED IN THE LIQUOR TO COMPLETE THE BLEACHING AND COMPLETING THE BLEACHING OF THE PARTICULAR MATERIAL WHILE REMAINING IN THE LIQUOR BY ACTIVATING THE CHLORITE FORMED THEREIN DURING THE PARTIAL BLEACHING. 